The proposed experments will use rats trained in a time discrimination task with food reward. The experiments study the effects of classical conditioning on time discrimination--in particular, how classical conditioning trials influence the timing of the conditioned stimulus (CS) by the internal clock used in time discrimination. They are based on earlier work that showed that the effective duration of a stimulus--the duration of the stimulus as measured by the animal--depends on the temporal contiguity of the stimulus and food. Some of the proposed experiments study in detail the changes in effective duration produced by changes in contiguity. For example, is the change in effective duration graded or dichotomous? Most of the proposed experiments, however, treat effective duration as if it is a new conditioned response and determine the parameters that control it and how those parameters exert their effects, just as one might study the determinants of, say, autoshaping. The parameters studied are the duration of the CS on conditioning trials, the probability that the CS ends with food, the density of food outside the CS, and the intertrial interval. The work will add to the understanding of classical conditioning and time discrimination, and to the larger question of how animals predict the future.